The Safety and Side Effects of Ivermectin in Dogs and Cats

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Ivermectin is used to treat a variety of different illnesses and is safe for most pets. However, side effects are possible. Photo Courtesy of FreeWine/Flickr.com.

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Ivermectin is commonly used in both dogs and cats for a variety of different diseases. It is used to treat various type of parasitic infections. Both internal and external parasites are frequently treated successfully with Ivermectin. In addition, it is used in many commonly available heartworm prevention medicines, such as Heartgard Plus® and others.

Safety of Ivermectin in Dogs and Cats

In many cases, the safety of Ivermectin is directly related to the dosage administered. As with many drugs, higher dosages tend to have higher risks of complications and potential side effects associated. Ivermectin is used in many dosage ranges, depending on the purpose of its usage. Dosages used for preventing heartworm infections are generally relatively low, with little risk of side effects.

Higher dosages, such as those used to treat demodectic mange, sarcoptic mange, ear mites, ​and other parasitic infections, are more likely to be associated with adverse reactions. However, for most dogs and cats, Ivermectin is considered to be a relatively safe medication when used appropriately.

How Ivermectin Works

The purpose of Ivermectin is to kill parasites. It does this by causing neurological damage to the parasite. This damage results in paralysis and death to the parasite, thus eliminating it from the pet's body. There are some dog breeds that are genetically sensitive to the medication. Their genetic mutation allows the Ivermectin to pass through the dog's blood-brain barrier. It then enters the dog's central nervous system, which can be fatal to the animal.

Side Effects of Ivermectin in Cats

In cats, Ivermectin has a fairly high margin of safety. When seen, side effects include:

  • Agitation
  • Crying
  • Lack of appetite
  • Dilated pupils
  • Paralysis of hind legs
  • Muscle tremors
  • Disorientation
  • Blindness
  • Other neurological signs, such as head pressing or wall climbing

If your cat is receiving Ivermectin and you notice these types of symptoms, discontinue the medication and contact your veterinarian.

Side Effects of Ivermectin in Dogs

In dogs, the risk of side effects associated with Ivermectin depends on the dosage, on the susceptibility of the individual dog, and on the presence of heartworm microfilaria (a larval form of the heartworm.)

When used at a low dose for heartworm prevention in a dog free of heartworms, Ivermectin is relatively safe. At higher doses which may be used for treating other parasitic infections, the risk of side effects increases. Potential side effects include:

  • Vomiting
  • Dilated pupils
  • Muscle tremors
  • Blindness
  • Incoordination
  • Lethargy
  • Lack of appetite
  • Dehydration

When used in a dog infected with heartworms, a shock-like reaction believed to be caused by dying microfilaria can occur. This type of reaction may be accompanied by lethargy, a low body temperature, and vomiting. Dogs testing positive for heartworms should be observed closely for at least 8 hours following the administration of Ivermectin.

Ivermectin Sensitivity in Collies and Similar Breeds

Neurotoxicity can also occur with Ivermectin usage in some dogs. This is particularly common in dogs that have a genetic mutation known as the MDR1 (multi-drug resistance) gene mutation. This gene mutation is known to occur most commonly in breeds such as Collies, Australian Shepherds, Shelties, Long-haired Whippets, English Sheepdogs, German Shepherds, Silken Windhounds, Skye Terriers, and other breeds with "white feet." Symptoms of neurotoxicity include incoordination, muscle tremors, seizures, blindness, and death.

Ivermectin used at dosages used for heartworm prevention is generally safe for these dogs. However, the drug should not be used at higher doses for dogs that may possess the MDR1 gene mutation. There is a test that can be performed to check for the gene mutation.

Ivermectin Toxicity

Ivermectin toxicity is very serious. It cannot be reversed. Speak to your vet right away if you believe your pet is suffering from an adverse reaction. They may recommend inducing vomiting or administering charcoal for the intent of minimizing absorption. 

Ivermectin and Human Use

In addition to its use in the veterinary medicine world, Invermectin is also used by doctors to treat a number of health issues in people. In humans, it is also useful in eliminating parasites. Ivermectin can be used to treat head lice, scabies, river blindness, and other conditions. It is only available with a prescription and is usually applied topically or taken orally.